Weed Biology & Ecology

Amber Vallotton

Horticulture Agent, VCE-Rockingham County

What makes a

weed a WEED?

Weeds Re-defined!

A weed is a weed because it possesses

certain definable characteristics that

set it apart from other plant species.

Weeds are plants that interfere with the

growth of desirable plants and that are

unusually persistent and pernicious.

They negatively impact human activities

and as such are undesirable.

Applied Weed Science, 1999

Weeds Re-framed • I want to offer a different CONTEXT & FRAMEWORK:

Thinking of weeds in terms of their BIOLOGICAL and

ECOLOGICAL CHARACTERISTICS

Defining Characteristics • Interference with growth of desirable plants

• Persistence

• Capacity to occupy sites disturbed by

human activities

• Injurious and/or destructive

Interference Interference is the total impact one plant

has another

Competition for light, water, and nutrients

• Can emerge before other desirable plants do, so tap

into critical resources

• Rapid population establishment

Interference Growth Form and Growth Habit

• Extent of root development, height, leaf area,

amount of branching

• Shading

• Low-growing, rapidly spreading

• Plant growth rates

Interference Weed Density

• Capacity of weeds to proliferate in large numbers

quickly reduces desirable plant’s ability to take up

resources

• Huge effect of weed pressure on desirable plants

Interference Allelopathy

Eliminating competition by producing toxins that

adversely affect other plants

Defining Characteristics • Interference with growth of desirable plants

• Persistence

• Capacity to occupy sites disturbed by

human activities

• Injurious and/or destructive

Persistence • Ability to produce numerous, long-lived, and easily

transportable seeds, as well as utilizing adaptations

for spread

Seed-Related Characteristics

Production of large number of seeds

Weed Approximate Number Seeds per Plant

Yellow nutsedge 2,400

Giant foxtail 10,000

Jimsonweed 23,400

Common lambsquarters 72,000

Redroot pigweed 117,000

Black nightshade 178,000

Russian thistle 200,000

Persistence

Persistence Seed-Related Characteristics

• Production of viable seeds under adverse

conditions

Persistence Long- term seed survival (Seed dormancy)

• Ability to remain viable indefinitely

Weed Years Longevity

buried in Soil

Common milkweed 3

Shattercane 10

Common cocklebur 16

Foxtail 20

Field bindweed 20+

Johnsongrass 20

Canada thistle 21

Jimsonweed 40

Redroot pigweed 40

Canada thistle- 21y

Jimsonweed- 40y

Johnsongrass- 20y

Persistence Adaptation for short distance and long distance

dispersal

Persistence Alternative adaptations for spread through presence

of vegetative reproductive structures

• Serves as major food storage organs, as well as buds

capable of generating new plants

• Provides another structure of overwintering in addition

to seeds

• Allows another means for propagation besides seeds

• Allows extension to new sites for water and nutrients

• Allows quicker growth than plants developing from

seeds

• Mechanism for survival even if parent plant is

destroyed

Persistence

Johnsongrass

Persistence Bermudagrass

Persistence Nutsedge

YNS & PNS-

Two of the

world’s worst

weeds!!!

Persistence

Bamboo

Defining Characteristics • Interference with growth of desirable plants

• Persistence

• Capacity to occupy sites disturbed by

human activities

• Injurious and/or destructive

Defining Characteristics • Capacity to occupy sites disturbed by human

activities

• All the things already described make weeds well

suited and more competitive to establish in sites

where non-weedy species cannot do so

Defining Characteristics • Interference with growth of desirable plants

• Persistence

• Capacity to occupy sites disturbed by human activities

• Can be injurious and/or destructive

What makes a weed a weed?

Defining Biological Characteristics

• Interference

• Persistence

• Capacity

• Destructive/Injurious to animals and crops

Weed Life Cycles

and Plant Anatomy

Annual Life Cycle Herbaceous Plants (non-woody aerial stems)

• Annuals (one growing season)

Annual Life Cycle Herbaceous Plants (non-woody aerial stems)

• Annuals

Pigweeds, Jimsonweed,

Ragweed, Morningglories Foxtail, Crabgrass, Barnyardgrass,

Shattercane, Fall panicum

Biennial Life Cycle Herbaceous Plants (non-woody aerial stems)

• Biennials (require two growing seasons)

Biennial Life Cycle Herbaceous Plants (non-woody aerial stems)

• Biennials

Common mullein, Musk thistle,

Wild carrot, Wild parsnip

Perennial Life Cycle Herbaceous Plants (non-woody aerial stems)

• Perennials (live indefinitely)

Perennial Life Cycle Herbaceous Plants (non-woody aerial stems)

• Two types perennials

• Simple (reproduce by seed & seed dispersal)

• Creeping (reproduce by both seed and vegetative

structures) [More in the next few slides…]

Dandelion, Plantain,

Pokeweed, Chickory

Perennial Life Cycle Creeping Perennials

Creeping root systems (Canada thistle, kudzu)

Bulbs (Wild garlic)

Perennial Life Cycle Creeping Perennial Structures

Rhizomes (Johnsongrass)

Perennial Life Cycle Creeping Perennials

Tubers (nutsedge)

Perennial Life Cycle Creeping Perennial Structures

Stolons (Bermudagrass)

Perennial Life Cycle Woody Plants (woody aerial stems)

Trees (perennials w/ single main stem/trunk)

Shrubs (perennials w/ more than one

principal stem)

Tree of heaven

Multiflora rose

Japanese

honeysuckle

Brief Look at Weed

Control Strategies

& Techniques

To find effective weed control,

you must take biological,

morphological, and life cycle

characteristics into account!!

STEPS Proper Weed Identification

• Weeds of the Northeast

• VT Weed Identification website

http://www.ppws.vt.edu/weedindex.htm

Determine Biological and Morphological

Characteristics and their influence on resistance to

control methods • Plant life cycle

• Seasonality Spring annual or summer annual?

• Growth habit?

• Does weed spread by seed? Vegetative structures? Both?

• Optimal times to use control methods rosette stage, before

seeding, active growth time, any time?

Weed Control Strategies & Techniques

Use this knowledge base to strategize best course of

action

Cultural Methods • Weed Suppression

Fast growing crop plants and close plant spacing

• Timely Rotations

• High-seeded Cover crops

Over winter (hairy vetch, rye w/ oats)

Pre-season full bed technique (to smother weeds)

Summer (buckwheat)

• Physical Barriers to prevent weed germination

Thick mulches in pathways and common areas

• Organic mulches like straw, wood bark/chips, newspaper

• Landscape fabric (breathes), plastic (does not breathe)

Weed Control Strategies & Techniques

Mechanical Methods

• Rapid removal of emerging weed seedlings

• TIMELY cultivating and hoeing to remove weeds

(check out Johnny’s Seeds) http://www.johnnyseeds.com

• Hand weeding

• Flame weeders

Remember weeds w/ creeping root systems may be

exacerbated by chopping and breaking them up!

Chemical Methods

• Herbicides (refer to Home and Grounds PMG for

control recommendations)

Girded with this

knowledge, you will never

look at weeds

the same way!!!